We will focus on the generalization of current optical models to allow for the accurate processing of FDPM data in a broader number of clinical situations. Specifically, we are interested in making FDPM measurements to probe layered tissue structures as well as assess the properties of small tissue volumes. In both these applications, we require FDPM measurements to be made at small source-detector (S-D) separations. However, it is well known that such measurements, when processed using algorithms based on standard optical diffusion theory (SODT), can lead to the determination of inaccurate optical values. Our goals are to generalize SODT to provide governing equations which accommodate spatially distributed collimated spaces and to solve these new equations for steady and amplitude modulated collimated point source located within an infinite medium. We will compare these results to solutions derived using SODT and experiment. We will also solve the new equati ons for s teady and amplitude modulated collimated sources illuminating the surface of an infinite medium. Compare results with SODT and experiment. Finally, we will develop a theoretical framework to address cases where FDPM measurements contain significant contributions from both minimally scattered and fully diffuse photons.